This invention generally relates to electrical contact assemblies, and, more specifically, to an improved explosion proof contact assembly and method of forming the same.
Contact assemblies are frequently used in hazardous locations in which quantities of flammable vapors, gases or the like exist in the surrounding atmosphere to create potentially explosive mixtures. It is, accordingly, extremely important in such hazardous locations to use explosion proof electrical devices which assure that any sparks created at electrical contacts are prevented from entering the potentially explosive atmosphere.
Electrical devices which are intended to be used in hazardous locations must, accordingly, be approved for the intended use.
An important factor in determining suitable uses of an electrical device in hazardous locations is the flame path. The flame path is normally considered to be the joint between two surfaces through which gases can travel from an area where an explosion can occur to where there can be combustible gases. It is desirable, therefore, to maintain the flame path at an adequate value to assure that sparks or hot gases become sufficiently cooled off as they travel along the flame path so as to become incapable of igniting explosive mixtures at the other end of the flame path.
The explosion proof flame path in existing receptacles is generally created by molding into the assembly separate metallic inserts for each contact. These inserts must be of sufficient length or contain sufficient grooves, ridges or other irregularities along the outside length to provide the required flame path. The inserts must be either solid or blind drilled and tapped. After the inserts are molded into the insulating body, the front and rear electrical contacts, which mate with the plug and line contacts, must then be assembled by either staking or fastening the contacts to the metallic inserts with screws. One example of an electrical device which utilizes long electrical conductors to provide the required flame path is the BHR insulator manufactured by the Crouse-Hinds Company.
The Crouse-Hinds Company has also made a contact assembly designated the ENR receptacle which was constructed so that the flame path requirements were met by having sufficient length of joint, from the front to rear, between the outside surface of the brass contact inserts and the surrounding molded plastic. After the insulator was molded, with the inserts molded in, the front, three-pronged contact and the rear, right-angled copper strap and button were then assembled to the contact inserts by placing them in the proper position and fastening them in place by staking over the ends of the contact inserts. The staking operation for the front, three-pronged contact required the staking tool to slide between the two parallel prongs of the contact in order to accomplish the staking. The tool, if not perfectly centered on the contact, could hit one of the prongs and push it down almost flat. Even when centered, there was no consistency where the tool would strike the insert. When the insert was staked off-center, a weak, unacceptable joint resulted. Both of the conditions mentioned resulted in complete loss of the insulator. The molded part could not be salvaged. In the situation where the tool was centered and the staking tool produced a good joint between the contact and the insert, the blade of the staking tool usually spread the two parallel prongs of the contact apart, necessitating manual rework to bend them back into position. The post-molding assembly operations used in the manufacture of the ENR receptacle were extremely difficult and expensive.
Other prior art constructions of electrical devices for use in hazardous locations are disclosed in the following U.S. Pat. Nos.: 3,235,682; 3,281,560; 3,394,338; 3,723,724 and 3,860,315. In U.S. Pat. No. 3,235,682, a connector is described where the flame path is formed by the contact/insulator interface which is designed to conventional standards of flame path lengths for axial joints. The rear insulator also must be potted after the molding process to insure contact spacings. The connector construction disclosed in U.S. Pat. No. 3,281,560 is not a complete molded assembly, but rather a sandwich construction with a resilient disc which is compressed by rigid thermoset plastic discs on either side. Explosion proof integrity is obtained primarily by inducing vaporization of a sublimable solid to generate flame extinguishing gases within the arcing chamber. The explosion proof electrical connector disclosed in U.S. Pat. No. 3,394,338 is designed for low energy, less than 500 watts, applications. The flame path is formed by a sandwich of three plastic discs and is only of the order of one-sixteenth of an inch in length with a clearance on the order of five-thousandths of an inch. The flame path configuration does not follow established standards for explosion proof flame paths. U.S. Pat. No. 3,723,724 discloses a safety mounted explosion proof light fixture. When the lamp section is removed from the mounted section, the contacts break at a point where the enclosure formed by the housing and the threaded joint with the lamp section still has the explosion proof integrity due to a minimum of five threads still engaged. The explosion proof connector which forms the subject matter of U.S. Pat. No. 3,860,315 has conventional flame paths through the joints of the insulating members and incorporates a telescoping housing to form the arcing chamber and depends upon threaded joints of the telescoping housing for cooling of gases escaping to the atmosphere.
Generally, the electrical devices disclosed in the above-identified patents are complex in construction and, therefore, expensive to manufacture. In some instances, the devices are impractical or ineffective for the purposes intended.